This comparator implementation provides a subset of the features provided
by the Unix/GNU Sort. In particular, the supported features are:
-n, (Sort numerically)
-r, (Reverse the result of comparison)
-k pos1[,pos2], where pos is of the form f[.c][opts], where f is the number
of the field to use, and c is the number of the first character from the
beginning of the field. Fields and character posns are numbered starting
with 1; a character position of zero in pos2 indicates the field's last
character. If '.c' is omitted from pos1, it defaults to 1 (the beginning
of the field); if omitted from pos2, it defaults to 0 (the end of the
field). opts are ordering options (any of 'nr' as described above).
We assume that the fields in the key are separated by
MRJobConfig.MAP_OUTPUT_KEY_FIELD_SEPERATOR.

setKeyFieldComparatorOptions

keySpec - the key specification of the form -k pos1[,pos2], where,
pos is of the form f[.c][opts], where f is the number
of the key field to use, and c is the number of the first character from
the beginning of the field. Fields and character posns are numbered
starting with 1; a character position of zero in pos2 indicates the
field's last character. If '.c' is omitted from pos1, it defaults to 1
(the beginning of the field); if omitted from pos2, it defaults to 0
(the end of the field). opts are ordering options. The supported options
are:
-n, (Sort numerically)
-r, (Reverse the result of comparison)